For stated preference (SP) studies, we develop a model that assesses the influence of choice set misspecification arising from the omission of perceived substitutes among real-world alternatives in the same class of goods. This problem is most likely to be present when individuals are allowed to select a “no purchase” option instead of being forced to choose from an explicit set of SP alternatives with hypothetical attribute levels. A convenient feature of our model is that researchers do not need to know exactly which omitted real substitute the individual most prefers, only the set of real substitutes that exists. In our empirical illustration, a comparison of rival models suggests researchers who overlook the presence of perceived real alternatives related to an SP experiment can end up with noticeably biased welfare estimates. Our more-general model suggests that it may be prudent for future SP researchers to anticipate, then test and possibly correct for, distortions in utility parameter estimates that result from this problem.